Portable AEDs have saved many lives in non-hospital settings and, as a result of advances in AED technology, the number of lives saved per year is rising. Typically, a portable AED analyzes a patient's heart function and instructs an operator to administer an electrical shock if appropriate. For example, a shock can often revive a patient who is experiencing ventricular fibrillation (VF). Because older models of portable AEDs include only basic diagnostic and safety features, they are often difficult to operate. Therefore, only specially trained persons such as emergency medical technicians (EMTs) can use these older models to administer shocks. Newer models, however, often include advanced diagnostic and safety features that allow minimally trained persons to administer shocks. Consequently, more people are using portable AEDs to save lives.
Because a heart condition that responds to an electrical shock can cause permanent damage or death within a short time if left untreated, a portable AED should be able to diagnose a shockable heart condition and be ready to shock a patient within seconds. Without cardiopulmonary resuscitation (CPR), a person in cardiac arrest will typically suffer permanent anoxia-induced brain damage within 4-6 minutes from the onset. Unfortunately, many people do not know how to administer CPR. And, even in the best of circumstances, it can take 1-4 minutes to retrieve the AED and 1-2 additional minutes to attach the pads to the patient, connect the pads to the AED, and activate the AED. Therefore, even if the patient is discovered immediately, the AED often has less than a minute to diagnose and shock the patient before he/she is in danger of suffering permanent brain damage. Clearly, the faster the AED can diagnose and shock the patient, the better the chances that the patient will survive with no permanent brain damage.
Unfortunately, many portable AEDs implement heart-analysis techniques that require a relatively long time to analyze the patient's ECG and to make a shock/no-shock decision based on the analysis.
FIGS. 1 and 2 illustrate contiguous windowing, which is a heart-analysis technique used by many portable AEDs. For example, referring to FIG. 1, a portable AED (not shown in FIG. 1) samples and analyzes contiguous “windows”, i.e., sections 10a-10f, of a patient's ECG. Typically, the AED individually analyzes multiple ECG sections 10, compares the respective analysis results to one another or to predetermined comparison values, and makes a shock/no-shock decision based on this comparison.
Referring to FIG. 1, an AED (not shown in FIG. 1) using contiguous windowing often requires a relatively long time to make a shock/no-shock decision. For example purposes, assume that the AED is programmed to analyze at least ten ECG sections 10 before making a decision, and that each section 10 is two seconds long. Therefore, the AED requires a minimum of twenty seconds to make a shock/no shock decision. Even though twenty seconds may not seem like a long time, every second required to make a shock/no-shock decision decreases the chances that a patient will survive with no permanent damage.
In addition, changes in the patient's heart function may increase the time that the AED requires to make a shock/no-shock decision. For example purposes, assume that before the AED can make a shock/no-shock decision, it is programmed to analyze ECG sections 10 until at least a predetermined number of ten sequential sections give consistent analysis results. The AED then bases its shock/no-shock decision on one or more of these consistent analysis results. This decision-making process is often called “voting”. The theory behind voting is that if a predetermined percentage of analyzed ECG sections yield consistent, i.e., similar results, then these results are more likely to be accurate than inconsistent results yielded by other ECG sections. For example, an AED may be programmed to accept the result yielded by the majority of analyzed ECG sections and ignore different results from the minority of analyzed ECG sections. In the illustrated example, the ECG section 10a indicates that the patient's heart is beating with a normal sinus rhythm, but the sections 10b-10f indicate that the patient is in VF. Therefore, because the analysis results obtained from the ECG section 10a will clearly be inconsistent with the results obtained from the sections 10b-10f, the AED must analyze at least seven ECG sections—the inconsistent section 10a plus at least six (a majority of ten) consistent sections starting with the section 10b—before making a shock/no-shock decision. If the six ECG sections starting with the section 10b are inconsistent, however, then the AED must analyze more ECG sections 10. Thus, the AED requires a minimum of fourteen seconds to make a shock/no-shock decision in this situation. Furthermore, although in this example the transition from normal sinus rhythm to VF occurs near the boundary between the ECG sections 10a and 10b, the same problem often arises when the transition occurs within a section 10.
Still referring to FIG. 1, one way to reduce the time that an AED requires to make a shock/no-shock decision is to shorten each of the ECG sections 10. For example, assuming that the AED is programmed to analyze at least ten sections 10 as discussed above, reducing the length of each section 10 from two seconds to one second reduces the minimum decision time from twenty to ten seconds. As the lengths of the ECG sections 10 decrease, the chances of an AED making an incorrect shock/no-shock decision increases. Specifically, as their lengths decrease, each of the sections 10 represents a smaller portion of the ECG. If a section 10 is too small, it does not contain enough ECG information to support an accurate analysis of the section. If all the sections 10 are too small, the AED makes a series of inaccurate analyses that may cause the AED to make an inaccurate shock/no-shock decision.
Another way to view this problem is as a tradeoff between section length and the number of sections. For example, for a given analysis time, e.g., 20 seconds, one can use longer sections (better accuracy per section) with fewer results to vote from (less voting accuracy) or shorter sections (less accuracy per section) with more results to vote from (more voting accuracy).
In addition, referring to FIG. 2, even when the ECG sections are not too short, an AED (not shown in FIG. 2) using contiguous windowing may incorrectly diagnose a patient's heart condition, and thus may determine that a defibrillating shock will benefit a patient when in actuality the shock may harm the patient. In the illustrated example, the patient is experiencing bradycardia, which is characterized by abnormalities in the patient's QRS wave and by an abnormally low heart rate. Unfortunately, shocking a patient experiencing bradycardia is at best useless and at worst can send the patient into VF or cause other cardiac damage. Therefore, it is important that the AED recognize bradycardia and other unshockable heart conditions and generate a no-shock decision if it determines that a patient is experiencing any of these conditions.
More specifically, if a boundary, i.e., the beginning or end, of an ECG section 12 intersects an important part of the ECG, then the AED's analysis of that section may yield an incorrect diagnosis, and the AED may make an incorrect shock/no-shock decision based on this incorrect diagnosis. In the illustrated example, the AED analyzes contiguous ECG sections 12a, 12b, 12c, which are each one and a half seconds long. Unfortunately, the beginning of the section 12a intersects a QRS complex, and thus the section 12a contains only part of the complex. Because there are no other full complexes within the section 12a, the AED's analysis of the section 12a may yield an incorrect result. But if the ECG sections 12b and 12c and a predetermined number of following sections 12 respectively include full QRS complexes, the AED can use voting to ignore the result from the section 12a and correctly make a no-shock decision as discussed above. Although as discussed above this may increase the time that the AED requires to make a shock/no-shock decision, the AED makes a correct decision. Conversely, if the ECG sections are shortened, e.g., to 0.5 seconds in order to obtain a quicker response, a majority of the sections will be lacking a QRS complex. These sections may be incorrectly interpreted as benefiting from a shock, resulting in an inappropriate shock diagnosis.
Still referring to FIG. 2, there are currently no analysis techniques for overcoming the intersecting-boundary problem other than to vote among multiple contiguous ECG sections, thereby delaying diagnosis, or to have a skilled operator (not shown in FIG. 2) study the ECG and determine if the AED's shock/no-shock decision is correct.
Therefore, the need has arisen for a heart-condition analysis technique that is faster and more accurate than the contiguous-window analysis technique.